Electrical power transmission and distribution equipment is subject to voltages within a fairly narrow range under normal operating conditions, and the equipment may operate at high voltages of, for example, 1000V or greater. However, system disturbances, such as lightning strikes and switching surges, may produce momentary or extended voltage levels that greatly exceed the levels experienced by the equipment during normal operating conditions. These voltage variations often are referred to as over-voltage conditions. If not protected from over-voltage conditions, critical and expensive equipment, such as transformers, switching devices, computer equipment, and electrical machinery, may be damaged or destroyed by such over-voltage conditions and associated current surges. Accordingly, it is routine practice for system designers to use surge arresters to protect system components from dangerous over-voltage conditions.
A surge arrester is a protective device that is commonly connected in parallel with a comparatively expensive piece of electrical equipment to divert over-voltage-induced current safely around the equipment, thereby protecting the equipment and its internal circuitry from damage. The surge arrester normally operates in a high impedance mode that provides a low current path to ground having a relatively high impedance. In this mode, normal current at the system frequency is directed to the electrical equipment and is prevented from following the surge current to ground along the current path through the surge arrester. When exposed to an over-voltage condition, the surge arrester operates in a low impedance mode that provides a high current path to electrical ground having relatively low impedance. When the surge arrester is operating in the low-impedance mode, the impedance of the current path is substantially lower than the impedance of the equipment being protected by the surge arrester. In this mode, current from the over-voltage condition is directed to ground and not to the electrical equipment. Upon completion of the over-voltage condition, the surge arrester returns to operation in the high impedance mode. The surge arrester also includes a disconnector that disconnects the surge arrester from ground if the over-voltage condition is too extreme or continues too long.
FIG. 1 is a partial cross-sectional view of a conventional high voltage surge arrester 90. As illustrated in FIG. 1, the high voltage surge arrester 90 typically includes an elongated outer enclosure or housing 100 made of an electrically insulating material, a pair of electrical terminals 102, 104 at opposite ends of the enclosure 100 for connecting the arrester between a line-potential conductor (not shown) and electrical ground (not shown), respectively, and a stack or array of other electrical components 106 that form a series electrical path between the terminals 102 and 104. Terminal studs 108, 110 connect to the line and ground terminals 102 and 104, respectively. An insulated mounting bracket or hanger 114 also may be provided for mounting of the arrester 90 to, for example, another piece of equipment or to a utility pole.
To prevent short circuiting of line potential conductors connected to the surge arrester 90, a disconnector 112 is provided on the ground terminal stud 110. The disconnector 112 may include an internal resistor or other electrical element connected in parallel with a spark gap assembly and a charged black powder in an unprimed .22 caliber cartridge that is heat activated. Thus, in the event of a sustained over-voltage current flow through the terminal stud 110, a spark is generated by the spark gap assembly of the disconnector 112. Heat from the spark detonates the charged powder cartridge to mechanically sever electrical connection between the terminal stud 110 and the lower terminal 104 in the housing 100, thereby isolating the terminal stud 110 from the line connection. The force created by the activation of the charged powder cartridge typically causes the terminal stud 110 to separate from the surge arrester 90, thereby effectively isolating the failed arrester from the power system.
Undesirably, portions of the heat sensitive disconnector 112, including the terminal stud 110, can become a projectile when the cartridge is inadvertently exposed to heat during shipping, transit, or storage. During transport and storage, if an accident or other occurrence results in a fire near one or more arresters, activation of the charged powder cartridges of the disconnectors in the arresters can be hazardous to first responders at the scene of the fire. Projectiles attributable to detonation of the charged powder cartridges of the disconnectors in such circumstances are of particular concern, particularly when a large number of arresters with such disconnectors are shipped and stored. A variety of different types of conventional surge arresters with disconnectors are vulnerable to the hazards noted above. Additionally, similar problems may be experienced by all disconnector devices. The problems noted above are therefore not considered unique to any particular disconnector or to any particular surge arrester.
In light of the hazards posed by arresters when subjected to a fire during shipping, transit, or storage, the United States Department of Transportation (DOT) has classified conventional surge arresters as hazardous materials that must be transported in accordance with DOT hazardous material transportation regulations. Transporting arresters under those guidelines increases the cost of such transportation. Alternatively, the DOT safety regulations can be met by fitting arresters with restraints that prevent the terminal stud and portions of the disconnectors from becoming projectiles when the disconnector cartridge is inadvertently exposed to heat during shipping, transit, or storage. However, adding such restraints increases the cost of arresters. Another option is to package the arresters in sturdy metallic cases during shipping, transit, or storage to meet the DOT requirements, though such packaging may be prohibitively expensive.
Accordingly, a need in the art exists for a surge arrester disconnector that is not classified as a hazardous material under DOT regulations.